1. Field of the Invention
The present invention generally relates to voltage converters and regulators and, more particularly, to voltage converters and regulators having several switches or other devices connected in series to form a voltage divider. The circuits in accordance with the present invention are particularly useful for providing low voltages at high currents needed for modern microprocessors. The non-isolated bus converters are suitable for high power density, high efficiency non-isolated unregulated DC/DC conversion but can be operated to provide good voltage regulation, as well.
2. Description of the Prior Art
As microprocessor technology advances, the required microprocessor power supply must provide power with lower voltage, higher current, and higher slew rate. Next generation microprocessors will require operating power at unprecedented low voltages and high current. For example, microprocessors may soon require voltages as low as 0.7 Volts and currents as high as 130 Amps or more with extremely wide and rapid excursions of current load between full operational mode and so-called standby or sleep states. This power must typically be generated from a DC power bus providing 48 volts or 12 volts. Providing reliable, small size, and cost effective power supplies and power conditioning circuits to meet the power requirements of state of the art microprocessors is increasingly difficult. In fact, present state of the art DC-DC converter circuits and voltage regulators will soon be inadequate for use with many foreseeable microprocessor designs.
Present state of the art voltage converters or regulators typically have a single stage or multiple stage buck converter. Two stage buck converters tend to reduce switching losses and reverse recovery losses in the buck converter switches. Also, in a two-stage buck converter, the reduced input voltage to the second stage allows the operating frequency of the second stage to be greatly increased. Consequently, the size of filtering capacitors can be greatly reduced. However, the efficiency gains and size reductions achievable from adding buck stages are inherently limited. Two stage buck converters may not be able to meet the power requirements of microprocessors in a cost effective circuit of acceptably small size.
On the contrary, the rate of change of current that can be delivered to a load, referred to as slew rate, is limited not only by the transient performance of the power supply/voltage regulator (hereinafter sometimes referred to simply as VR or voltage regulator module (VRM)) itself but by interconnect parasitics which can be excessive and further limit transient performance. Therefore, use of filter capacitors of increasing values and size have been used for energy storage to meet needed slew rates even though the limited control bandwidth of VRs is compromised by such increased capacitance.
It has been recognized that the requirement for capacitors to accommodate high slew rate of load current can be reduced by operating switching voltage regulators at higher switching rates or frequencies to provide increased control frequency and transient response. However, such an option has not been widely adopted because the efficiency of known voltage converter topologies is significantly reduced when switching frequencies are increased because the major loss factors are switching losses and body conduction losses associated with the inherent diode in MOSFETs which are used for switching.